It is presently believed that the apparatus and process according to the present invention will have broad application in the field of food processing and perhaps beyond that field. For example, there are problems in drying rice, roasting nuts and coffee that may be overcome by the apparatus and processes disclosed herein. Nevertheless, the novel apparatus and processes disclosed herein are known to offer a number of advantages which are peculiar to coffee roasting. Accordingly, the initial development efforts have been directed to that field and the description of the preferred embodiments of the invention will emphasize coffee roasting without in any way limiting the broader aspects of the invention. Other applications will be readily apparent to those who are skilled in the art of treating particulate material.
In its simplest form, coffee roasting comprises heating a single bean to prescribed temperature at which point chemical reactions occur that transform the bean into the desired state of pyrolysis. These reactions occur in the last part of the heating cycle. Thus, the residence time at the terminal temperature is crucial because a difference in a few seconds in heat-history can have a significant effect on the taste of the coffee.
The problem is that it is difficult to design a roaster that will roast several hundred pounds of beans at one time and to roast every bean evenly. Whether the process for heat transfer is from convection, conduction, radiation, or some combination thereof, the heat is absorbed in the first few layers of a bean bed. Therefore, it is desirable to establish some means for equalizing bean temperature throughout the heating cycle so that when the final roasting temperatures are approached, all of the beans will be close to the same temperature during the pyrolysis process.
The prior art is replete with attempts to obtain roasting uniformity. For example, various approaches for roasting coffee are set forth in the U.S. Pat. No. 2,857,683 of Schytil.
In the aforementioned prior art processes, the heating time to reach critical temperatures were considered to be relatively unimportant. For example, prior art processes typically roasted coffee beans for periods of six to twenty minutes. However, in recent years, it has been found that coffee beans expand more and result in lower roast bean density if the heating process is speeded up to where the total heating cycle is accomplished in as short a time period as possible consistent with acceptable product characteristics, preferably within 70-90 seconds. Further, it has been found that these light density beans, when ground, have increased extractable solids and wettability, thus yielding an increase in extractable solids when employing conventional time and temperature brewing devices. The result of fast roasting is that coffee processors can fill the traditional 16 ounce container with a much reduced weight of coffee that still results in an equivalent number of cups as 16 ounces resulting from a longer roasting process.
Therefore, it is presently believed that there is a significant demand for an apparatus and/or process which will raise the coffee bean temperature to a specified point, maintain a more uniform temperature across a bed of beans and complete the roast in a time period which is almost an order of magnitude shorter than conventional roasting of a few years ago. It is also believed that such apparatus will have broad application for roasting and drying vegetable products and for treating other materials.
One approach to the more rapid roasting of coffee beans is disclosed in the U.S. Pat. No. 4,737,376 of Brandlein et al. As disclosed therein, the beans have a residence time within the roaster for a period of much less than three minutes and perhaps less than 1.5 minutes. During roasting, the beans are subjected to a flow of heated gas which passes upwardly through a first foraminated container at a mass flow rate of at least ten pounds of gas per pound of beans. In that process, the depth of the expanded bed is less than 50% of the diameter of the container. Further apparatus for the fluidized bed roasting of coffee is disclosed in the U.S. Pat. No. 3,964,175 of Sivetz. The Sivetz disclosure also contains a survey of prior art fluid bed roasters.
The efforts to obtain faster roasting have for the most part relied on the use of a fluidized bean bed and hot air. However, attempts to drive the requisite amount of air needed for fast heating through the bed causes the bed to become unduly levitated and change into a spouting bed. This undue levitation and spouting results in a substantial loss in heating efficiency. Also, the individual beans in such systems are thrown about in a random fashion which adversely affects the uniformity of the roast.
Another approach for roasting coffee beans uses a downblast of hot air into the beans instead of fluidization. This approach, like fluidization, produces random bean movements and results in a lack of bean uniformity. For example, such roasters have been found to produce coffee having several color units of variation because the beans are blown backward as well as forward and therefore receive different amounts of heat.
There is one further consideration for roasting coffee beans and for heating and/or drying particulate vegetable material. In some cases, a continuous roaster is favored. Such roasters are typically very large in size and capable of roasting 10-12,000 pounds per hour. Thus, the machines take up a large amount of floor space, are suitable for large processing plants and are relatively inflexible. For example, such machines are not usually readily changed over for producing different roasts or the like. Batch machines, on the other hand, are more appropriate for a majority of roasting shops which produce a plurality of products or blends. The reason is that many coffee processors operate like a typical job shop where there are many changes during the day of blends, type of roast, degree of roast, etc., with relatively short runs of each. In addition, the smaller shops do not generally need the large capacity of a continuous roaster.
Thus, it appears that there is a need for an improved apparatus and method for uniformly conditioning particulate material. It is believed that there is a need for an improved apparatus and method for uniformly roasting batches of coffee very rapidly and with an efficient use of energy. It also appears that there is a demand for improved conditioning, cooling, heating and roasting apparatus which is relatively flexible, competitively priced, relatively simple in operation, free of complexity and easy to operate and maintain. Also, it appears that there is a demand for improved apparatus and methods which will occupy relatively small area and which can be rapidly converted to operate under different conditions in a job shop type of operation while fulfilling all of the requirements for food processing.
It is presently believed that the apparatus and methods to be described hereinafter will meet most, if not all, of the aforementioned criteria.